This invention relates to a ship hull adapted for operation in ice-filled waters.
As a ship moves through an ice field, chunks of ice in front of the ship are forced below the surface of the water by virtue of the ship's forward movement. The chunks of ice then slide along the external suface of the underwater part of the hull. Some of the chunks of ice slide along the bottom surface of the hull, thereby easily obstructing the ship's propeller. This reduces propeller efficiency, and the chunks of ice can also cause damage to the propeller. The chunks of ice which slide along the side surfaces of the ship can also get into the ship's propeller, particularly in twin or triple propeller ships.
The purpose of the invention is to create a favorable hull form for a ship operating in icy waters, so that chunks of ice will be directed in such a way as to have as small an adverse effect as possible on the forward movement of the ship.
The configuration of the hull of a ship is conveniently illustrated by means of its so-called frame form, which shows the external cross-section of the hull at a defined location along the length of the ship. Several frame forms may be illustrated in a single figure (such as FIG. 1 of the accompanying drawings). The main frame form is the frame form midway along the hull's design waterline length.
For convenience of nomenclature, the "side surfaces" of the hull are those portions of the hull, extending downwards from the top of the hull at port and starboard respectively, which are vertical or are inclined to the vertical at a smaller angle than to the horizontal, and the "bottom surface" of the hull is that portion of the hull, beneath the side surfaces, which is inclined to the horizontal at a smaller angle than to the vertical.
It will be appreciated that in order to maximize the internal volume, or cubic capacity, of a ship's hull, within given overall dimensions, the hull is normally constructed so that its middle section is of substantially uniform beam, and the hull's bottom surface, within the middle section, is made as wide as possible within the limits determined by the sides of the ship.
According to a first aspect of the present invention, there is provided a ship having a hull defining a bow having a substantially V-shaped form in the waterline plane of the ship, and having a general frame form defining a bottom surface and two side surfaces, the bottom surface of the hull having a bottom section that is horizontal in a cross-section of the hull and extends longitudinally of the hull midway between the two side surfaces of the hull, and the bottom surface of the hull also having two inclined bottom sections that are located entirely below said design waterline plane and are joined to the horizontal bottom section along respective sides thereof and to the side surfaces respectively, each such join being along a clearly defined longitudinal intersection edge, whereby each side of the hull is divided into at least three clearly defined portions, the width of said horizontal bottom section of each of the inclined bottom sections being at least 20% of the maximum beam of the hull, and the inclined bottom sections extending over a substantial part of the length of the hull.
According to a second aspect of the present invention there is provided a ship hull defining a bow having a substantially V-shaped form in the design waterline plane of the hull, and having a general frame form defining a bottom surface and two side surfaces, the bottom surface of the hull having a bottom section that is horizontal in a cross-section of the hull and extends longitudinally of the hull midway between the two side surfaces of the hull, and the bottom surface of the hull also having two inclined bottom sections that are located entirely below said design waterline plane and are joined to the horizontal bottom section along respective sides thereof and to the side surfaces respectively, each such join being along a clearly defined longitudinal intersection edge, whereby each side of the hull is divided into at least three clearly defined portions, the width of said horizontal bottom section and of each of the inclined bottom sections being at least 20% of the maximum beam of the hull, and the inclined bottom sections extending over a substantial part of the length of the hull.
In the case of a ship embodying the first aspect of the invention, or a ship having a hull embodying the second aspect of the invention, very few chunks of ice go under the horizontal bottom section of the hull because it is quite narrow. Owing to their buoyancy, the chunks of ice which go under the inclined bottom section move towards the sides. Since the bow of the ship is substantially V-shaped in the waterline plane, the bow's main section has a wedge-shaped form in horizontal cross section, and this pushes the ice to the sides. However, it is not essential to the invention that the very front part of the bow at the design waterline be sharp. The bow can be designed in the manner described in U.S. Pat. No. 4,351,255.
In a preferred embodiment of the invention, which is intended for operation in extremely difficult ice conditions, the lower part of each side surface is inclined. It is also important for the horizontal bottom section and the inclined bottom section together to extend over a considerable proportion of the length of the ship. In practice this means that the length of that part of the ship which has the defined general frame form extends over at least 20% and preferably at least 30% of the ship's design waterline length. It is most desirable that the defined general frame form applies to the form of the main frame of the ship. If the ship has a middle section of uniform beam whose length is at least 25% of the ship's waterline length, it is preferred that the defined general frame form should apply at a point whose distance to the rear from the foremost point of the ship's middle section of uniform beam is 20% of the design waterline length, provided that this point is forward of the main frame.
As indicated previously, if the horizontal bottom section is made too narrow, the hull will have an unfavorable cubic capacity because the ship's internal volume is small in relation to the ship's overall dimensions. Owing to this, it is most advantageous for the horizontal bottom section to be at least 30% of the ship's maximum beam. If the ship only has a single propulsion propeller, the width of the horizontal bottom section should preferably be at least as large as the diameter of the propulsion propeller. If twin propellers are used for the ship's propulsion, the total width of the horizontal bottom section should preferably be at least 80% of the distance between the axes of these propellers. The horizontal bottom section may be of uniform beam for at least part of its length forward of the point where the bottom line of the ship begins to rise towards the stern of the ship. The horizontal bottom section may even become wider towards the stern. In the latter case, a more favorable hull form in terms of cubic capacity is obtained and at the same time more effective steering of the chunks of ice towards the sides is achieved. The horizontal bottom section may then advantageously be narrow at the forward narrowing section of the ship, for instance having a width of no more than 15% of the maximum beam of the ship. At the point of the bow section, where the bottom line of the hull proper of the ship rises in the forward direction, the horizontal bottom section may be at the bottom of a wedge-shaped structure protruding downwards from the hull proper of the ship.
If the ship has side surfaces that are inclined to the vertical, the angle of inclination should not be made too large because the hull's cubic capacity is then adversely affected.
The angle of inclination of the side surfaces is preferably smaller than the angle between the inclined bottom section and the horizontal plane. This too favorably affects the guiding of the chunks of ice. The angle of inclination of the inclined bottom section in relation to the horizontal plane is preferably between 5 and 30 degrees, more preferably from 8 degrees to 20 degrees. In a particularly preferred embodiment of the invention, the side surface has an inclined section that extends upwards from the outer edge of the inclined bottom section so far above the ship's design waterline that its end point remains above the waterline even when the ship heels to the maximum angle for which the ship is designed. The inclined bottom section preferably extends in the longitudinal direction of the ship both backwards and forwards at least to the point where the ship's bottom line begins to rise and in the aft direction preferably extends a considerable distance farther than that point.
In a hull embodying the invention it is very advantageous to use the air bubbling system described in U.S. Pat. No. 3,580,204. Air discharge openings are located at the lower edge of the inclined bottom section, whereby the hull form enhances the beneficial effect of the bubbling and the bubbling also enhances the beneficial effect of the hull form. It is advisable to provide the air discharge openings over that longitudinal section of the hull for which the ship's bottom line is substantially horizontal. It is also advantageous to arrange air discharge openings forward of this section. This can easily be done, if the bow section of the ship has a structure that extends downwards from the hull proper as has already been described. A hull form with an inclined side surface below the waterline best enhances the effect of air bubbling.